The SimRiver Simulation, developed by Dr. Shigeki Mayama’s lab at Tokyo Gakugei University, models human impact on water quality of a river. Settings within the simulation help students to make sense of this phenomenon by allowing them to mimic changes in land use along the river, population size, the presence/absence of sewage treatment, and season of the year. Using diatoms as indicator species of water quality, students identify diatoms from water samples, determine the saprobic index (a measure of water quality), and compare the impact of different human land and water uses along the river. As students become familiar with the simulation, they can use it to plan investigations and explore possible solutions to mitigate adverse impacts of human actions on water quality, and thus biodiversity. Instructional supports include background information, instructions for using the simulation, a printable student worksheet, and three levels of student support through different challenge settings.